This invention relates to synthetic lubricants which are a blend of alkanoic esters of hindered polyhydric alcohols having 3 to 8 hydroxyl groups and polyols or polyol ethers.
Rotary screw air compressors are well known in the art as can be seen from U.S. Pat. Nos. 2,622,707 (12-23-'52); 3,073,513 (1-15-'63); 3,073,514 (1-15-'63); and 3,129,877 (4-21-'64).
It is well known to use hydrocarbon lubricating oils to seal the rotors of the foregoing rotary screw air compressors, lubricate the bearings and cool the compressed gases. Due to the high temperature and pressure of the air, it has been found that these hydrocarbon oils break down and create a sludge in a relatively short time, i.e. about 1000 hours or less.
In attempts to lengthen the intervals between changing out the lubricants, resort has been made to the use of silicone fluids. These silicone fluids such as Sullair's 24-KT are very expensive and represent a considerable capital investment in that a new compressor unit with different bearings and seals is required. The use of carboxylic acid esters of polyols is known from U.S. Pat. No. 4,175,045 dated Nov. 20, 1979. Carboxylic acid esters of polyols are considered not sufficiently viscous at 210.degree. F. to be effective in a rotary screw compressor.
It is known that synthetic esters made from dicarboxylic acids have been used to produce long lasting compressor fluids, such as Anderol 495 sold by Tenneco. The major component of Anderol is considered to be a dialkyl adipate. However, it is known that these synthetic esters are not hydrolytically stable. Anderol 500 (a dialkyl phthalate composition) is also known to be useful in reciprocating air compressors. However, this fluid is specifically recommended for reciprocating air compressors and is considered to be too viscous at low temperatures for use in rotary air compressors.
In U.S. Pat. No. 4,072,619 (dated Feb. 7, 1978) polyester-alkylene glycol compositions are disclosed wherein phenothiazine is incorporated into the alkylene glycols. However, these compositions have been found to degrade in a relatively short time i.e. 1000 hours.
Synthetic lubricants comprising a major amount of a polyester and a minor amount of a monocapped polyglycol are known from British Pat. Nos. 933,721; 986,066; and 1,162,818, however these compositions are disclosed to be only useful in aircraft gas turbines where gross contamination with water is not a problem.